DF's claims isn't that it doesn't have any noticeable effect in performance (in fact in a recent video[0] they claim the opposite when Denuvo was introduced to Assassin's Creed Mirage after reviews were done - it can affect performance and thus because of that introducing it after the fact invalidated the CPU performance metrics reviewers had done) but - at least in the cases i remember - they cannot tell if the issues were due to Denuvo or not.
Note though that in a case where they could test both versions, they did find Denuvo to both have lesser performance and affect frame pacing[1] (though that is for a 2yo game, things might be better more recently).
